We have done a fair amount of magnesium repairs over the past few years.  I had a seasoned welding veteran bring some fabricated magnesium intakes from a pulling tractor over last week for repair.  No problem, we did the repair and that was that.  While he and I were shooting the breeze he told me of an old trick to tell if a material is aluminum or magnesium.  Take a lemon and squeeze some of the juice onto the surface, it it's aluminum it will not do anything, where as if it's magnesium it will buddle up like peroxide.  I assume if this is true, it must be the citric acid that reacts with the magnesium.

Anybody used this method before?